The present invention relates generally to diagnostic imaging and, more particularly, to a method and apparatus of dynamically filtering radiation emitted toward a subject during radiographic imaging.
Typically, in radiographic imaging systems, an x-ray source emits x-rays toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” may be interchangeably used to describe anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-rays. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
In computed tomography (CT) imaging systems, the x-ray source and the detector array are rotated about a gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-rays as a beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and a photodiode for receiving the light energy from an adjacent scintillator and producing electrical signals therefrom. Typically, each scintillator of a scintillator array converts x-rays to light energy. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data processing system for image reconstruction.
There is increasingly a need to reduce radiation dosage projected toward a subject during an imaging session. It is generally well known that significant dose reduction may be achieved by using a “bowtie” filter to shape the intensity profile of an x-ray beam. Surface dose reductions may be as much as 50% using a bowtie filter. Generally, the bowtie filter is constructed such that the profile used to filter the radiation projected toward the subject is constant through the imaging process. That is, the bowtie filter is constructed to filter radiation emitted toward a subject such that the sum of the radiation path length through the filter plus the path length through the subject is roughly constant. One drawback of this construction is that a subject's shape is not constant. For example, a patient's shape changes as a function of view angle as the x-ray tube and detector rotate around the patient and because the filter has a constant filtering profile at different points in the imaging process, portions of the subject may be over-radiated whereas at other times, other portions will be under-radiated. As a result, a choice must be made between having under-radiated portions during the imaging process or increasing the dosage such that a minimum radiation dosage throughout the imaging session is maintained with some portion being over-radiated.
A number of developments have been made to address this situation. The CT scanner may be equipped with multiple beam shaping filters. Multiple filters, however, increases the manufacturing costs of the CT scanner as well as requiring additional controls to insure the appropriate filters are being used during the scan session. Another approach is to dynamically control the amount of x-ray dosage emitted by the projection source as a function of view angle. This approach, however, requires significant and complex controls to alter the amount of radiation projected toward the subject during the scan that significantly increases the costs of the CT system nor takes into account shape of the pre-patient filter.
Therefore, it would be desirable to design a pre-subject collimator having a filter that rotates and effectively changes its filtering and been shaping properties as a function of view angle as the x-ray tube and detector rotate around a subject.